WHAT DO PRAMS AND TIGERS HAVE IN COMMON?

Prams of the big, old-fashioned kind had overlapping wheels. Why? Overlapping wheels can be larger in diameter for the same length of chassis. Larger wheels have less rolling resistance, meaning that they need less energy to roll over the same terrain. Imagine a small-wheeled push chair meeting a brick; now imagine a big-wheeled pram meeting the brick.

The Tiger had roadwheels of 800 mm in diameter – a major jump on the Panzer IV’s 500 mm. The Tiger II had wheels of the same diameter. The Challenger main battle tank today has wheels no larger. The Tiger’s wheels were surpassed by only remarkable Panther tank’s 860-mm wheels – the largest on any medium tank of the Second World War. The Soviet T34 had wheels of a diameter in between, but its wheels were not overlapping.

History of overlapping wheels

A set of Tiger wheels from underneath

Overlapping the wheels allows you to cram more wheels and larger wheels into the same length of chassis. Germany was the only belligerent of the Second World War to deploy vehicles with overlapping wheels. It had deployed half-tracks with such technology before the war. The idea for a tank with overlapping wheels goes back to at least 1935, when Daimler-Benz, Krupp, and MAN all offered designs for a medium tank with interleaved road wheels and torsion bar suspension.

However, the winning bid (eventually the Panzer IV) was similar to the current medium tank (Panzer III), with several small wheels, tightly packaged as pairs, each pair on a leaf spring. In 1940, both the Panzer III and Panzer IV were tried experimentally with large overlapping wheels, although the technology was applied to production of the Panzer II (from 1941) and the Panzer I (from 1942).

The Tiger was designed in 1941 with the same technology (at least, the winning bid by Henschel was; the bid by Porsche had smaller wheels packaged in pairs, each pair on a longitudinal torsion bar).

Tiger Wheel Arrangement

The Tiger’s arrangement allowed for four rows of wheels each side, each row of four wheel-units, for a total of 16 wheel-units each side. Compare the Porsche bid’s 6 smaller roadwheels each side (which can be seen today at The Tank Museum on the Ferdinand/Elefant self-propelled gun).

The Elefant’s wheels for comparison.

The Tiger’s wheels on the two central rows were doubled, in the sense that each was composed of two wheels fixed together. Counting these separate components gives an amazing count of 24 wheels each side.

Counting the wheels (as well as factoring in their diameter and their width) gives a fairer measure of the weight distribution, because each of those wheels is spreading the load on the ground. By simply adding a wheel on the outside of an existing wheel, you halve its load (you’ve probably seen agricultural tractors with added wheels).

A wider track helps to spread the load from the wheels above, but this effect diminishes as the track reaches wider beyond the roadwheels (think of the Soviet KV and JS heavy tanks, with wide tracks, but small, narrower wheels). Although historians often talk about wide tracks as the only explanation for low ground pressure, a better measure is the width of distribution of the load on the wheels.

The Tiger’s wheels were technically not just overlapping but also interleaved, meaning that some of those roadwheels were contained between rows of other wheels. The first, third, fifth, and seventh wheel arms each carry roadwheels on the outermost row and the third row in, on the latter of which the roadwheels are doubled, for a total of three roadwheels each arm. The second, fourth, sixth, and eighth arms carry roadwheels on the second row and the fourth (innermost) row, of which the former are doubled.

Disadvantages

Tiger 131’s road wheels were removed completely during restoration

The disadvantages of interleaved arrangements are expense, mud or ice clogging between roadwheels, and the difficulty of replacing roadwheels on the inner row. These disadvantages were mitigated on the Tiger II by staggering roadwheels in two rows rather than interleaving four rows. The Tiger II had as many roadwheels in total by adding an extra arm each side and doubling the wheels on every wheel unit. However, this solution imposed inverse bending loads on the track links.

Consequently, in the subsequent peace, where sustainability becomes more important, no main battle tank has been deployed with overlapping roadwheels, such that most main battle tanks actually are inferior to the Tiger in ground pressure and soil flotation.

Don’t take my word for it: see for yourself The Tank Museum’s Tiger 131 running on Tiger Day, smoothly caressing the surface better than much lighter and more modern vehicles.

Tiger Day 11 & 12 | 27.04 & 15.09 2019

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